Combustion exhaust is a regulated emission and various exhaust aftertreatment devices located in the engine exhaust play a role in detection and controlling of exhaust emissions. Diesel particulate filters (DPF) are commonly used for particulate matter (PM) or soot control, and lean NOx traps (LNT) are used for NOx control. Under lean conditions, an LNT adsorbs oxides of nitrogen (such as nitric oxide NO and nitrogen dioxide NO2, also referred to as NOx for short) produced during engine combustion. Lean-burn engine exhaust contains oxides of sulfur (SOx), derived from fuel and lubricating oil, which compete with NOx for LNT adsorption sites. Unfortunately, SOx is preferentially adsorbed over NOx and forms stable sulfates with the LNT storage materials. As a result, LNT performance gradually declines because fewer storage sites are available for NOx adsorption.
To effectively manage sulfur poisoning of LNT, high temperature desulfation is periodically performed on the LNT. Desulfation requires a high temperature exhaust and cycles of lean and rich conditions to release sulfur from the LNT adsorption sites. However, the hydrogen sulfide (H2S) gas released as a byproduct of desulfation introduces a harsh chemical environment for the various sensors and detectors located downstream of the LNT in the exhaust line. For example, soot sensors located downstream of the LNT, may get degraded in the harsh chemical environment. Typically, resistive-type soot sensors estimate an exhaust soot level based on a correlation between a measured change in electrical conductance (or resistance) between a pair of interdigitated comb electrodes of the sensor with the amount of PM deposited between the measuring electrodes. However, H2S released as a by-product of desulfation may react electrochemically on the sensor electrodes, thereby corroding the electrodes and reducing the sensitivity of the soot sensors. For example, the reaction of the H2S with the soot sensor electrode may cause the sensor gain to drift.
Various approaches have been developed for protecting soot sensor electrodes from corrosion resulting from H2S released as a by-product of LNT desulfation. One example approach is shown by Berger et al. in U.S. Pat. No. 7,543,477. Therein, the soot sensor electrodes are covered with a protective layer manufactured from an electrically insulating base like aluminum oxide or zirconium dioxide and further doped with a conductive material, such as a metal or graphite. The protective layer may serve to protect the soot sensor electrodes from direct exposure to the harsh chemical environment encountered in the exhaust line.
However, the inventors have recognized potential issues with such an approach. As one example, adding additional protective layer may reduce the electrostatic attraction between the charged soot particles and the soot sensor electrodes and may lead to reduced soot sensor sensitivity. With reduced sensitivity, the soot sensor may not be able to determine the leakage of the particulate filter in a reliable way. Thus errors in the sensor may lead to a false indication of DPF degradation and unwarranted replacement of functioning filters.
The inventors herein have observed that H2S appears to preferentially react with the negative electrode of the soot sensor. Specifically, following H2S reaction, the degree of corrosion on the negative electrode was significantly higher than the degree of corrosion on the positive electrode. In view of this observation, the inventors have recognized that corrosion of the soot sensor negative electrode can be reduced by making the negative electrode temporarily appear like the positive electrode. Specifically, during conditions when the level of H2S in the exhaust is high, such as during desulfation of the LNT, by making the negative electrode have an open circuit floating potential, with no possibility of electron flow through the negative electrode, the selective corrosion of the negative electrode may be reduced.
Thus in one example, corrosion of soot sensor electrodes may be addressed by a method for selectively connecting and disconnecting a positive electrode of interdigitated comb electrodes of a soot sensor to/from a positive voltage and selectively connecting a negative electrode of the interdigitated comb electrodes of the sensor to ground via a measuring resistor. In this way, by varying the coupling of the electrodes of a soot sensor to a positive voltage based on exhaust conditions, H2S-induced corrosion of soot sensor electrodes is reduced.
As one example, the circuitry of a resistive-type particulate matter sensor may be adjusted to include a three-way switch coupled to the negative electrode of the sensor. Based on exhaust conditions, a position of the three-way switch may be adjusted so that the negative electrode is coupled to one of the positive voltage source of the sensor, to ground, or left open. At the same time, a two-way switch may couple or decouple the positive electrode of the sensor to the positive voltage. When the soot sensor is collecting particulate matter in the exhaust, the method includes selectively connecting the positive electrode to the positive voltage (by closing the two-way switch connecting the positive electrode and the positive voltage) and selectively connecting the negative electrode to ground via a measuring resistor (by shifting the three-way switch connecting the negative electrode to the positive and ground to a first position). During desulfation of the LNT, when the exhaust H2S levels are high, the soot sensor is operated first in a pre-desulfation (prior to desulfation of the LNT) mode and then followed by a desulfation mode. During pre-desulfation of the LNT, the positive electrode may be selectively disconnected from the positive voltage (by opening the two-way switch) and the negative electrode may be selectively connected to the positive voltage (by shifting the three-way switch to a second position). Following this, during desulfation of the LNT, the positive electrode may be maintained disconnected from the positive voltage and additionally the negative electrode may be selectively disconnected from the positive voltage (by shifting a three-way switch to a third or open position).
The technical effect of coupling the negative electrode first to the positive voltage during pre-desulfation, and then disconnecting it during desulfation from both the positive voltage and the ground via a three-way switch of the sensor is that when H2S is released during desulfation, the negative electrode transiently resembles the positive electrode, and that both the sensor electrodes act as open circuit floating potentials, with reduced possibility of electron flow. During other conditions, such as soot sensor regeneration or LNT regeneration, by adjusting the switch, the negative electrode may be disconnected from both the positive voltage and the ground. This reduces the preferential electrochemical reaction between the released H2S and the negative electrode. As a result, soot sensor gain drift is reduced, thereby reducing soot sensor corrosion during LNT desulfation. By reducing the gain drift of the soot sensor, sensor accuracy is improved, lowering the risks for false indication of particulate filter degradation. Furthermore reducing gain drift of the soot sensor better enables detection of polluting exhaust for PMs. As such, this reduces the high warranty costs of replacing functional particulate filters and exhaust emissions are improved and exhaust component life is extended.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.